The present invention relates to a technique effective when applied to adjustment of a change in a circuit characteristic due to variations in external electronic parts or devices of a semiconductor integrated circuit (hereinbelow, also written as IC), particularly, adjustment of a frequency shift of an oscillator having an oscillating element. More particularly, the invention relates to a technique effective when applied to an IC for communication control of, for example, a Bluetooth communication standard and a communication system using the IC.
One of wireless communication standards is the standard called Bluetooth for a short-distance wireless communication using a frequency band from 2.4 GHz to 2.48 GHz. A wireless communication system conformed with the Bluetooth standard (called Bluetooth system) is generally constructed by an RF IC having functions of transmitting/receiving signals and modulating/demodulating signals, a baseband LSI (Large Scale Integrated circuit) for performing a baseband process for generating, for example, I/Q signals on the basis of transmission data and a baseband process on reception data, and generating RF IC control signals, and a program memory including nonvolatile memories such as a ROM (Read Only Memory) for storing a control program (user program) and an EEPROM (Electrically Erasable Programmable Read Only Memory).
In the Bluetooth system, a control for mutual communications with a device as the party on the other end is executed by a function circuit called a link controller. In the Bluetooth communication, to synchronize clocks and also establish communication connection between a master device and a slave device, on the basis of a clock signal of 3.2 kHz called a Bluetooth clock, packet data is exchanged in cycles (625 μs) which is twice as long as 3.2 kHz.
When the mutual communication connection is established, each link controller calculates a deviation amount between the Bluetooth clock of itself and a Bluetooth clock of the device on the other end. The link controller on the slave side corrects the clock deviation and performs communication control. By the control, the communication operation on the master side and that on the slave side are synchronized with each other. In the Bluetooth standard, frequency precision of ±20 ppm from the Bluetooth clock of 3.2 kHz is demanded. Generally, the clock is generated by frequency-dividing a reference clock of 8 MHz, 13 MHz, or the like.
Hitherto, in the Bluetooth system, to generate a clock of 8 MHz, 13 MHz or the like as a reference clock, a quartz oscillator having relatively high precision is generally used among oscillators which are easily available. Even an oscillator using a quartz oscillator, however, has a problem such that the oscillation frequency is deviated from a desired value due to manufacture variations of the oscillator itself, in addition, parasitic capacitance of a mounting board, manufacture variations of capacitive elements connected together with the oscillator, a temperature change, and fluctuations in the power source voltage.
As a technique for correcting a frequency deviation in the oscillator using the quartz oscillator, an invention is provided in which, for example, a variable capacitance diode is connected to an oscillator, a nonvolatile memory (EEPROM) for prestoring trimming data for correcting variations detected by measurement is provided, the trimming data read from the nonvolatile memory is converted by a D/A converter to an analog voltage, the analog voltage is applied to the variable capacitance diode and, by changing the capacitance value, the oscillation frequency of the oscillator is adjusted to zero (refer to Japanese Unexamined Patent Publication No. Hei 10 (1998)-41746).